Piezo actuator

ABSTRACT

A piezoelectric actuator for actuating a mechanical component in which the piezoelectric actuator is provided with a multilayer construction of piezoelectric layers and can be acted upon by an electrical voltage in a piezoelectrically active region via inner electrodes disposed between the layers. At least one inactive region is present in the layer construction of the piezoelectric actuator, and this region is formed without inner electrodes, of a material whose mechanical and thermal properties match the properties of the active region, including the combination and the interaction of the material of the piezoelectric layers and of the inner electrodes.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a 35 USC 371 application of PCT/DE 03/01810 filed onJun. 3, 2003.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to a piezoelectric actuator, for instance foractuating a mechanical component such as a valve or the like.

2. Description of the Prior Art

It is widely known that by using what is known as the piezoelectriceffect, a piezoelectric element may be constructed from a material thathas a suitable crystal structure. When an external electrical voltage isapplied to these piezoelectric and electrostrictive ceramics, amechanical reaction of the piezoelectric element ensues, which as afunction of the crystal structure and the regions where the electricalvoltage is applied represents a pressure or tension in a predeterminabledirection.

Because of the extremely fast and precisely regulatable reciprocationeffect, such piezoelectric actuators may be contemplated forconstructing final control elements, for instance for driving switchingvalves in fuel injection systems in motor vehicles. The voltage- orcharge-controlled deflection of the piezoelectric actuator is utilizedfor positioning a control valve, which in turn regulates the stroke of anozzle needle.

Since the required electrical field intensities for actuating thepiezoelectric actuator are in the range of several kV/mm, and as a rulemoderate electrical voltages for triggering are desired, theconstruction of this piezoelectric actuator is done here in multiplelayers of metallized piezoceramics, stacked one above the other, to formwhat is known as a multilayer actuator. To that end, between each of thelayers there are inner electrodes, applied for instance by a printingprocess, and there are also outer electrodes by way of which theelectrical voltage is applied. One typical method for producing suchlayers is film casting. To produce the inner electrodes, the individuallayers are metallized and stacked one above the other, and then betweentwo layers, with inner electrodes of different polarity, thepiezoelectric effect develops.

At the top and bottom regions, however, the inner electrodes as a ruleare missing, since on the one hand, a certain insulation distance isneeded at the end faces, to avoid short circuits toward the outside, andon the other, passive zones are utilized for electrical connection ofthe outer electrodes. Passive regions may also be present inside theactuator. Often, however, in the zone between the region with innerelectrodes, as an active zone, and the region without inner electrodes,as a passive zone, there is the risk of cracks. This phenomenon shortensthe service life and can lead to the total failure of the actuator.

It is known per se from German Patent Disclosure DE 100 25 998 A1 thatpassive regions of variable length are disposed on both ends of the plyor layer construction. A total length of the piezoelectric actuatorfixedly defined for a particular installation situation is attained hereby way of reducing the length of the piezoelectrically inactive topand/or bottom piece, for instance by hard machining by means of grindingor the like.

In this known piezoelectric actuator, on the one hand, the passivelayers may be of the same ceramic material as the active region, butwith outer electrodes that are contacted electrically on only one sideor not at all, so that the inactive regions are also penetrated by themetal layers of the inner electrodes. On the other hand, the respectiveinactive region may also be a completely electrically insulated metal orceramic block, which can simply be glued to the piezoelectrically activeregion, for instance.

SUMMARY AND ADVANTAGES OF THE INVENTION

The piezoelectric actuator described above as noted, consists of amultilayer construction of piezoelectric layers and in apiezoelectrically active region with inner electrodes located betweenthe layers and is provided with contacting of the inner electrodes thatalternates from layer to layer, for subjecting it to an electricalvoltage. There is moreover at least one inactive region, such as abottom and/or top part, on one end or even inside the active region, inthe total installed length in the layer construction of thepiezoelectric actuator.

Advantageously, the piezoelectric actuator of the invention, in at leastone inactive region without inner electrodes, is formed of a materialwhose mechanical and thermal properties match the properties of theactive region, including the combination and interaction of the materialcomprising the piezoelectric layers and the inner electrodes. Examplesof matching mechanical and thermal properties of the piezoelectriclayers of the inactive region and of the active region that can beconsidered are the thermal expansion, the elasticity, and the shrinkageupon sintering of the multilayer construction of the piezoelectricactuator.

In an especially advantageous embodiment, in the piezoelectric actuatorof the invention, the inactive regions and the active region are madefrom an identical ceramic basic substance, with additional dopantsinserted into the inactive regions. Preferably, the basic substance islead zirconate titanate (PZT), and the dopant is silver. Silver isadvantageous in the sense that it is often incorporated into the activeregion, and the inner electrodes are therefore preferably constructed ofAgPd, so that when additional doping with silver is done in the passiveregion, similar properties are obtained.

With the invention, it is advantageously possible in particular tominimize the tendency of cracking in piezoelectric stacked actuators atthe transition from active to inactive regions. This is accomplishedprimarily in the production process, which directly minimizes thefailure rate. A primary cause of cracking here is that in sintering ofthe piezoelectric actuator, the ceramic in the region having the innerelectrodes shrinks to a different extent than in the region withoutinner electrodes. This is associated with the fact that chemicalelements of the inner electrode diffuse into the ceramic and dope it andthus change the shrinkage properties. The shrinkage mismatch then leadsto the damaging mechanical stresses.

The arrangement according to the invention moreover, however, promotesreliability during the operation of the piezoelectric actuator. Inoperation of the actuator, it expands in the active region because ofthe longitudinal effect (d₃₃ effect) in the longitudinal direction.Simultaneously, however, a transverse contraction occurs (d₃₁ effect).Since in the passive region the ceramic remains rigid, this differencein expansion also leads to mechanical stresses.

Moreover, the coefficient of thermal expansion differs between theactive and the passive region, because of the influence of the metal ofthe inner electrodes in the active region, so that mechanical stressesalso occur upon temperature changes. Thus the advantage of the inventionis above all that in the region between the active and inactive regions,the mechanical stresses are minimized, and all the causes mentionedabove are taken into account in order to avoid cracking.

BRIEF DESCRIPTION OF THE DRAWINGS

One exemplary embodiment of the piezoelectric actuator of the inventionis described herein below in conjunction with the sole figure of thedrawing, which shows a section through a piezoelectric actuator with amultilayer construction made up of layers of piezoceramic and of activeand inactive regions.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

In the drawing, a piezoelectric actuator 1 is shown, which in a mannerknown per se is constructed of piezoelectric layers 2 of a ceramicmaterial, for instance so-called green films, with a suitable crystalstructure, so that by utilizing what is known as the piezoelectriceffect, when an external electrical direct voltage is applied to theinner electrodes 3 and 4, via externally contacted electrodes notidentified by reference numeral here, a mechanical reaction of thepiezoelectric actuator 1 ensues.

The piezoelectric actuator 1 is subdivided into a piezoelectricallyactive region A and two inactive or passive regions B and C mounted onthe top and bottom ends, respectively. In this context, a region iscalled active that is penetrated by alternating polarity from the innerelectrodes 3 and 4 and in the final analysis thus contributes to thetotal longitudinal expansion, which is wanted for operation, of thepiezoelectric actuator 1.

In the exemplary embodiment of the invention, the multilayerpiezoelectric actuator 1 is constructed in such a way that thecomposition of the material of the ceramic layers differs in the activeregion A from the passive regions B and C. The difference is designedsuch that then, taking into account the combination and interaction ofthe material of the piezoelectric layers and of the inner electrodes inthe region A, the mechanical and thermal properties of the layers of theinactive region B, C and of the active region A, and in particular thethermal expansion, the elasticity, and the shrinkage upon sintering ofthe multilayer construction of the piezoelectric actuator, arecompatible.

Thus for producing the green films, intrinsically two types of ceramicare necessary. These may for example be types of ceramic that areprepared differently, for instance on the basis of lead zirconatetitanate (PZT). This or other ceramics on a commonly based basicsubstance can be varied and adapted accordingly by adding suitabledopants, such as silver.

The ceramic of the respective passive region B, C is compounded in sucha way that the properties with respect to shrinkage upon sintering,thermal expansion, and elasticity are ideally compatible with theceramic of the active region A, in combination with the inner electrodesand the resultant interaction.

Thus from these two ceramics, two types of green film can be produced,and in stacking to form the so-called green body of the piezoelectricactuator 1, films of the type for the region B and the region C may thenbe used for the top and bottom regions B and C and optionallyintermediate regions, not shown here. For the active region or regions,green films for the active region A are used; in that case, these greenor ceramic films need merely be metallized in order to produce the innerelectrodes.

The foregoing relates to a preferred exemplary embodiment of theinvention, it being understood that other variants and embodimentsthereof are possible within the spirit and scope of the invention, thelatter being defined by the appended claims.

1. In a piezoelectric actuator, comprising a multilayer construction ofpiezoelectric layers (2) and inner electrodes (3, 4), which are locatedin a piezoelectrically active region (A) between the layers and can beacted upon by an electrical voltage, and at least one inactive region(B, C) without inner electrodes in the layer construction of thepiezoelectric actuator (1), the improvement wherein the at least oneinactive region (B, C) without inner electrodes is formed of a materialwhose mechanical and thermal properties, including shrinkage uponsintering of the multilayer construction of the piezoelectric actuator(1), match the mechanical and thermal properties of the active region(A), including the combination and interaction of the materialcomprising the piezoelectric layers (2) and the inner electrodes (3, 4),wherein the inactive regions (B, C) and the active region (A) are madefrom an identical ceramic basic substance, with additional dopantsinserted into the inactive regions (B, C), wherein the basic substanceis lead zirconate titanate (PZT), and the dopant is silver, and whereinthe matching mechanical and thermal properties of the piezoelectriclayers (2) of the inactive region (B, C) and of the active region (A),include thermal expansion and elasticity.